Along with the market demand for longer lives and better fuel economy of automobile engines in recent years, there is a demand for a piston ring for use in those engines to have a thin piston ring width, a light weight, a low tension, and a low friction loss. Here, a piston ring is mounted on a piston for realizing smooth reciprocating movement of the piston in a cylinder and sealing hot combustion gas. A piston mounted with a piston ring forms a combustion chamber in a cylinder, and an explosive pressure, which is generated by the combustion of fuel in the combustion chamber, is converted into rotation of a crankshaft. Accordingly, a piston ring to be mounted on a piston is required to have a function of stably sealing the hot combustion gas for a long period of time withstanding explosive shocks in the concerned combustion chamber. If such sealing performance by a piston ring cannot be improved, that may become a factor to cause a deterioration of engine output, an increase of oil consumption, and the like.
To improve the shock resistance and the gas sealing performance described above, there are various kinds of piston rings corresponding to features of engines, including those made of materials which can avoid abnormal wear during initial running-in, those having shapes which are effective in preventing blow-by (blowing through) of gas and are suitable from a viewpoint of lubrication theory, and the like. For example, in the case of pistons for diesel engines, as the combustion pressure of internal combustion engine rises due to recent emission regulation control, the load imposed on the piston has been increasing, and from the need to withstand such a severe use condition, the material for the piston ring groove has become steel or cast iron. Therefore, if a piston ring which is made of the same material as that of the piston ring groove is mounted, adhesive wear is likely to occur.
In order to suppress the occurrence of the concerned adhesive wear, a piston ring to be mounted on a piston for a diesel engine is configured to have a shape provided with an inclination angle, which is the same as that of the concerned piston ring groove, on the side of the top face and/or bottom face thereof. If adhesive wear occurs between the piston and the piston ring, the sealing performance between the top and bottom faces of the piston ring and the piston ring groove of the piston will degrade, becoming a factor to cause deterioration of engine output, increase of oil consumption, and the like.
To avoid the problems described above, for example, Patent Literature 1 (Japanese Patent Laid-Open No. 2004-92714) discloses a combination of piston and piston ring, which is excellent in durability, and is suitably used in a high power internal combustion engine involving a high combustion temperature and combustion pressure like a high-power engine. To be specific, the combination of piston and piston ring of Patent Literature 1 is configured such that a piston in which at least the piston ring groove is made of steel, is mounted with a piston ring which is made of cast iron and is formed with a hard coating at least on the outer peripheral sliding surface. Further, Patent Literature 1 discloses that making the piston ring out of cast iron will result in the formation of irregularities on the top and bottom faces of the concerned piston ring due to the presence of graphite peculiar to cast iron, so that these irregularities contribute to oil collection, as well as graphite acts as a self-lubricating substance, thus suppressing the occurrence of adhesion between itself and the piston ring groove made of steel material which is the mating member, thereby obtaining excellent durability.